Coordination Polymers in Dicyanamido-Cadmium(II) with Diverse Network Dimensionalities

: The synthesis and structural characterization of six dicyanamido-cadmium(II) complexes are reported: catena -[Cd( µ 1,3 -dca)( µ 1,5 -dca)(3-ampy)] ( 1 ), catena -[Cd 3 ( µ 1,3,5 -dca) 2 ( µ 1,5 -dca) 4 (pyNO) 2 (H 2 O) 2 ] ( 2 ), catena -{Cd(H 2 O) 2 ( µ 1,5 -dca) 2 ](2,6-lut-NO)} ( 3 ), catena -[Cd(Me 2 en)( µ 1,5 -dca) 2 ] ( 4 ), catena -[Cd(Me 4 en)( µ 1,5 dca) 2 ] ( 5 ), and [Cd(1,8-damnp) 2 (dca) 2 ] ( 6 ), where dca = dicyanamide anion, 3-ampy = 3-aminopyridine, pyNO = pyridine-N-oxide, 2,6-lut-NO = 2,6-lutidine-N-oxide, Me 2 en = N,N-dimethyl-ethylenediamine, Me 4 en = N,N,N (cid:48) ,N (cid:48) -tetramethyl-ethylenediamine, and 1,8-damnp = 1,8-diaminonaphthaline. The coordination polymers have different dimensionalities: 1 and 5 form 3D networks structures; 3 and 4 form polymeric 1D chains and 1DD double chains, respectively. Ribbons of three fused polymeric chains are observed in 2 . In 6 , the mononuclear complex units form a hydrogen-bonded supramolecular 3D network. In the coordination polymer compounds, the dca linkers display three bonding modes: the most common µ 1,5 -dca and the least popular µ 1,3 - and µ 1,3,5 -dca bonding. The luminescence emission and thermal properties of the complexes were investigated. (ON-2,6-lut); bidentate ligands, N,N-dimethyl-(Me 2 en) and N,N,N ′ ,N ′ -tetramethyl-ethylenedia-mine (Me 4 en); and 1,8-diaminonaphthalene (1,8-damnp), in addition to the ambidentate 3-aminopyridine ligand (3-ampy), which possibly can act as a bridging ligand, too. The structural formulas of these ligands are shown in Scheme 2. The selected ligands should provide vacant coordination sites at the cadmium center and, hence, allow its interaction with the dca linker(s) to propagate the formation of bridging coordination polymers. The photoluminescence emission and thermal properties of the complexes were also investi-gated. (ONpy) and 2,6-lutidine-N-oxide (ON-2,6-lut); bidentate ligands, N,N-dimethyl-(Me 2 en) and N,N,N (cid:48) ,N (cid:48) -tetramethyl-ethylenediamine (Me 4 en); and 1,8-diaminonaphthalene (1,8-damnp), in addition to the ambidentate 3-aminopyridine ligand (3-ampy), which possibly can act as a bridging ligand, too. The structural formulas of these ligands are shown in Scheme 2. The selected ligands should provide vacant coordination sites at the cadmium center and, hence, allow its interaction with the dca linker(s) to propagate the formation of bridging coordination polymers. The photoluminescence emission and thermal properties of the complexes were also investigated.


Experimental
Herein, we examine the interaction of Cd(II) and dca in the presence of a wide range of auxiliary N-donors ligands with variable skeletons simple monodentates, such as N-pyridyloxide (ONpy) and 2,6-lutidine-N-oxide (ON-2,6-lut); bidentate ligands, N,N-dimethyl-(Me 2 en) and N,N,N ,N -tetramethyl-ethylenediamine (Me 4 en); and 1,8-diaminonaphthalene (1,8-damnp), in addition to the ambidentate 3-aminopyridine ligand (3-ampy), which possibly can act as a bridging ligand, too. The structural formulas of these ligands are shown in Scheme 2. The selected ligands should provide vacant coordination sites at the cadmium center and, hence, allow its interaction with the dca linker(s) to propagate the formation of bridging coordination polymers. The photoluminescence emission and thermal properties of the complexes were also investigated. One of the very important property associated with (n-1)d 10 metal ions such as Cd is their tendency to produce photoluminescence emission [5,15,24,25,32], a phenomen which can possibly be used in photochemical devices [32][33][34][35] and in the catalytic activ of some organic reactions, such as Knoevenagel condensation [36]. In addition, ca mium(II) coordination chemistry and coordination polymers, as well as metal orga framework materials, were successfully employed in the crystal engineering architectu for the design of many interesting compounds, which showed diverse structural prop ties and promising applications [33][34][35][36][37][38][39][40].Herein, we examine the interaction of Cd(II) a dca in the presence of a wide range of auxiliary N-donors ligands with variable skeleto simple monodentates, such as N-pyridyloxide (ONpy) and 2,6-lutidine-N-oxide (ON-2 lut); bidentate ligands, N,N-dimethyl-(Me2en) and N,N,N′,N′-tetramethyl-ethylened mine (Me4en); and 1,8-diaminonaphthalene (1,8-damnp), in addition to the ambident 3-aminopyridine ligand (3-ampy), which possibly can act as a bridging ligand, too. T structural formulas of these ligands are shown in Scheme 2. The selected ligands shou provide vacant coordination sites at the cadmium center and, hence, allow its interacti with the dca linker(s) to propagate the formation of bridging coordination polymers. T photoluminescence emission and thermal properties of the complexes were also inve gated.

Experimental
Scheme 2. The structural formulas and abbreviations of ligands used in this study.

Synthetic Aspects and IR Spectra of the Complexes
With the high tendency of Cd(II) ion to predominantly form six-coordinate complexes, one expects that its interaction with the long dicyanamide anion (NCNCN) − (dca), in the presence of small non-sterically hindered mono-or bi-dentate co-ligands, to generate coordination polymeric compounds. With this hypothesis in mind, the reaction of a methanolic mixture containing Cd(NO 3 ) 2 .4H 2 O and ligands such as 3-ampy, NO-py, Me 2 en, and Me 4 en and an aqueous solution of Nadca in the stoichiometric ratio 1:1:2 afforded the expected CPs:  6), respectively. The former complex constitutes a CP, in which the 2,6-lutidine-N-oxide is not encountered in the coordination sphere of the complex, whereas, in 6, the dca is acting as a monodentate ligand. The purity of the isolated complexes was checked by X-Ray Powder Diffraction and the XRD method, and the graphs of these patterns are depicted in Supplementary Materials Figures S1-S6 for complexes 1-6, respectively. The complexes were structurally characterized by single-crystal X-ray crystallography, as well as elemental microanalyses and IR spectroscopy.
The IR spectra of the complexes reveal the general characteristic features of the dicyanamide group. In general, the complexes display three medium-strong intense bands over the vibration ranges 2290-2260, 2230-2220, and 2160-2150 cm −1 regions. The later band is attributable to ν s (C≡N), and the former two vibrational bands are attributable to ν as (C≡N) and ν s + ν as (C≡N), respectively [3][4][5][6][7]14,[26][27][28][29]48,49]. These bands are clearly pronounced in the complexes 1, 4, 5, and 6. The observed split of bands in complexes 2 and 3 is most likely attributed to the involvement of the aqua ligands in hydrogen bonds of the type O-H···N to N2 and/or N5 atoms of the adjacent dca groups (see Section 3.2). The further split of the bands in catena-[Cd 3 (µ 1,3,5 -dca) 2 (µ 1,5 -dca) 4 (pyNO) 2 (H 2 O) 2 ] (2) results from two dca coordination bonding modes. These two complexes also display medium broad bands over the range 3570-3340 cm −1 assigned for the ν(O-H) stretching frequency of the coordinated aqua molecules [50]. The weak-medium intense band(s) located over the 3280-3220 cm −1 region is/are due to the ν(N-H) stretching frequencies in complexes 1, 4, and 6. The weak ν(C-H) stretching frequencies were shown for all complexes over the 3100-2840 cm −1 region [50]. Representative IR spectra of compounds 1-3 are shown in Supplementary Materials Figures S7-S9.

Luminescence Emission
The photoluminescence emission of solid Cd-dca complexes were examined at room temperature. Unfortunately, the complexes 3-6 did not show any significant luminescence, whereas enhancement fluorescence emissions were observed in complexes 1 and 2, compared to their parent ligands and Nadca. Excitation of the two complexes at 366 nm revealed single emission maxima at 460 and 445 nm for 1 and 2, respectively. The corresponding ligands 3-ampy in 1 (Supplementary Materials Figure S13) and pyNO in 2 (Supplementary Materials Figure S14) showed a maximum intensity band at 429 and 454 nm, respectively. While a red shift was observed in the former complex, a small blue shift was detected in 2. The observed fluorescence enhancement in complexes 1 and 2 is most likely attributed to the increase of the conformational rigidity of the ligand upon coordination. The strong the overlap in the Cd-N (3-ampy) or Cd-O (pyNO) bond reduces the non-radiative decay within the intra-ligand (n-π*) excited state, hence enhancing the fluorescence intensity [6,7,55]. Thus, the non-radiative processes in complexes 3-6 are superior.

Thermal Analyses
The heating curves (TG and DSC) of the title compounds are presented in the Supplementary Materials section (Figures S15-S20), for 1-6, respectively. The heating curve of 2 shows a first step of weight loss of 3.51% (DSC signal at 121.0 • C), which corresponds to release of aqua ligand (Calculated 3.75%). The anhydrous product of 2 shows an explosive decomposition at 332.6 • C. The heating curve of 3 exhibits three narrow steps of weight loss, namely −2.52, −2.45, and −3.07% (sum 8.34%), with two resolved DSC peaks at 103.6 and 136.4 • C. These three steps of weight losses can be attributed to the release of the aqua ligands (Calculated 8.93%). The anhydrous title compounds show first steps of weight loss of 11.77, 0.83, 17.48, and 6.47%, with sharp DSC signals at 204. 1, 194.7, 197.9, and 185.3 • C, for compounds 1 and 4-6, respectively. The subsequent steps of weight loss at higher temperatures are accompanied by deflagration.